1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory which creates a localized level in high dielectric material so as to trap electrons for use as a block layer and a manufacturing method thereof.
2. Description of the Related Art
Generally, the flash memory which is one of the nonvolatile semiconductor memories is a nonvolatile memory which does not require electric holding operation (supplying holding power) for memory, and has been widely used in diversified electronic devices because programs and the like can be written easily after a product is completed. The NAND flash memory subsequent to next generation is demanded to be miniaturized further and operate at a lower voltage. The NAND flash memory subsequent to the next generation has been demanded to be formed in a flat cell because an inter-poly-dielectrics (IPD) cannot be buried between the cells easily. To secure the amount of electric charge due to shortage of a contact area between the IPD and the floating gate (FG) and achieve interference suppression between cells and lower voltage, technology for thinning the IPD, FG and tunnel film is indispensable.
However, if a local defect is generated in the tunnel film, most of accumulated electric charge is eliminated due to that defect because electric charges are accumulated on a metallic gate film. Thus, in case of the FG structure, the tunnel film needs to be thickened more than a specified level, which is a significant obstacle against miniaturization.
Instead of this FG structure, metal-oxide-nitride-oxide-semiconductor (MONOS) is a candidate as one of the memory cell structure having a discrete type charge accumulating layer. The memory cell containing the charge accumulating layer of the conventional MONOS has a structure in which a tunnel layer formed of an insulating film (oxide film), a trap layer formed of a silicon nitride film, a block layer formed of an insulating film and a control electrode are successively overlaid on a channel area of a silicon substrate in which for example source and drain are formed.
The block layer of the MONOS serves as a barrier for accumulating charges in the charge trap layer and needs to have a function of preventing charges from flowing out to the control electrode side. Additionally, the block layer needs to have a function of accumulating charges in the charge trap film and preventing electrons from flowing in from an electrode side. For that purpose, dielectric constant needs to be sufficiently high and the barrier against electrons needs to be sufficiently high.
However, a phenomenon that the electronic barrier of the general insulating film having high dielectric constant is low is a problem which is necessarily generated because the band gap of the high dielectric substance is small. To solve this problem, for example, JP-A 2006-270102 (KOKAI) has been proposed. This JP-A 2006-270102 (KOKAI) has proposed that the electronic barrier is raised by depositing negative charges in the block layer. This publication has a feature in adding negative charges by annealing. However, sufficient charges can not be deposited in a stable condition only by annealing.
Sufficient charges which can satisfy a performance required by the NAND flash memory subsequent to the next generation can be deposited stably and as a final structure, a nonvolatile memory with the sufficiently high electronic barrier by the deposited negative charges has been demanded.